1. Field of the Invention
This invention relates to a sheet-fed printing press, and more particularly to a plate cocking apparatus for correcting a register error resulting from distortion of a plate position.
2. Description of the Prior Art
In a multi-color printing press, a plate is wound on each of plate cylinders that are disposed in the same number as the number of colors, and printing is sequentially carried out for each color. In this case, high printing quality cannot be obtained unless each color printing is made sequentially at a correct position of a sheet of paper.
To obtain a correct register for each color, crisscross marks referred to as "register marks" are printed in a margin of the sheet of paper. A plate or a plate cylinder is register-adjusted by measuring the deviation of the register marks for each color.
Register errors can be classified into parallel errors where the register marks deviate in parallel in horizontal/vertical direction(s) of the plate, and distortion that occurs when phase error in the horizontal direction is combined with error in the vertical direction.
As a counter-measure for the parallel errors, a technique for correcting the register error by adjusting the plate cylinder itself in its axial/circumferential direction(s) has already been established but this technique is not the subject matter of the present invention.
As the counter-measures for the distortion, mention can be made of the manual adjustment technique which is disclosed in Japanese Patent Publication No. 53034/1988, but this technique wastes time and moreover, must stop the operation of a printing press. A cocking apparatus by socalled "plate cylinder cocking", which is disclosed in Japanese Patent Laid-Open No. 72731/1985, applies an excessive load to the printing press and moreover, its structure is extremely complicated.
Japanese Patent Laid-Open No. 56146/1987 discloses a hydraulic cocking apparatus for effecting so-called "plate cylinder cocking and Japanese Patent Laid-Open No. 108046/1989 discloses a technique which integrally moves a front grip end and a rear grip end of a plate. However, both of them are complicated in structure and are not much practical.